Numerous means have been sought to improve the fuel-efficiency of moving bodies and, especially, moving bluff bodies by reducing their aerodynamic drag. In the field of surface transportation, and particularly in the long-haul trucking industry, even small improvements in fuel efficiency can reduce annual operating costs significantly. It is therefore advantageous in the design of a vehicle to reduce drag forces, thereby increasing the aerodynamic properties and efficiency of the vehicle.
The over-the-highway cargo-hauling tractor-trailer combination is one vehicle that experiences excessive aerodynamic drag. Generally described, tractor-trailer combinations typically include a tractor having a so-called fifth wheel by which a box-like semi-trailer may be attached to the tractor by an articulated connection for transportation of the cargo trailer.
It is well known that bluff bodies, such as trailers, contribute significantly to aerodynamic drag, as evidenced by the formation of a wake along the length of and in the trailing region behind the trailer. The generation of the wake, formed by eddies, can be contributed to the shape of the conventional trailer, which is essentially a rectangular box having a flat, rectangular roof along with flat, rectangular side panels. The fore and aft vertical surfaces of such trailers are also generally flat rectangular surfaces. As such, current bluff bodies, such as the conventional trailer, which are generally suitable for use with tractors of Class 8 and other types, suffer from a low pressure zone at the rear of the trailer such that the air stream suffers from early separation, resulting in a broad eddying wake forming downstream of the separation. Trailers in motion also experience aerodynamic drag resulting from crosswind yaw conditions. Crosswinds result in vertical airflow up the sides of the trailers. The airflow separates from the top of the trailer, forming a strong cone vortex that increases drag and reduces vehicle efficiency. The net result is the creation of considerable aerodynamic drag.